Series

This animation shows a rotating 3-D image of a stickleback skeleton. The pelvic region, including the pelvic spines, is highlighted in red. Armored plating covers the flanks of the fish. The three prominent dorsal spines give the fish its name.

As a human embryo develops, its cells become progressively restricted in the types of specialized cells that they can produce. Inner cell mass (ICM) cells of the blastocyst can make any type of body cell. Gastrula-stage cells can give rise to the cells of a given germ layer. Later, cells become...